Ford's 3-speed automatic lineup includes the C4 and C6 — two of the most common and durable Ford automatics ever built. The FMX is a less common variant also noted here.
C4 (LIGHT DUTY): Configuration: 3-speed automatic Overall length: ~28" Input shaft: 26-spline (standard) or 24-spline (early versions) Weight: ~130 lbs Applications: 1964–1981 Ford and Mercury cars with smaller engines (inline-6, 289/302, 351W in some applications) Note: The C4 is a light-duty transmission — it is well-suited for small-block applications but is not rated for big-block torque levels.
C6 (HEAVY DUTY): Configuration: 3-speed automatic Overall length: ~30" Input shaft: 26-spline Weight: ~160 lbs Applications: 1966–1996 Ford and Lincoln cars and trucks, primarily with FE, 385-series, and 429/460 big-block engines; also used with some diesel and high-torque applications Note: The C6 is significantly more robust than the C4 and can handle the torque of big-block and diesel engines. Common in F-series trucks, Thunderbirds, and full-size cars with the 390, 428, 429, and 460 engines.
FMX: Configuration: 3-speed automatic Based on Ford's older MX/Merc-O-Matic design Used in 1968–1981 Ford cars — less common than C4 or C6 Not recommended for performance applications
C4 VS. C6 AT A GLANCE: • Both use 26-spline input shafts • C6 is about 2 inches longer and 30 lbs heavier • C6 is rated for significantly more torque • If the vehicle had a big-block, it has a C6
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